184 research outputs found
In-flight calibration of STEREO-B/WAVES antenna system
The STEREO/WAVES (SWAVES) experiment on board the two STEREO spacecraft
(Solar Terrestrial Relations Observatory) launched on 25 October 2006 is
dedicated to the measurement of the radio spectrum at frequencies between a few
kilohertz and 16 MHz. The SWAVES antenna system consists of 6 m long orthogonal
monopoles designed to measure the electric component of the radio waves. With
this configuration direction finding of radio sources and polarimetry (analysis
of the polarization state) of incident radio waves is possible. For the
evaluation of the SWAVES data the receiving properties of the antennas,
distorted by the radiation coupling with the spacecraft body and other onboard
devices, have to be known accurately. In the present context, these properties
are described by the antenna effective length vectors. We present the results
of an in-flight calibration of the SWAVES antennas using the observations of
the nonthermal terrestrial auroral kilometric radiation (AKR) during STEREO
roll maneuvers in an early stage of the mission. A least squares method
combined with a genetic algorithm was applied to find the effective length
vectors of the STEREO Behind (STEREO-B)/WAVES antennas in a quasi-static
frequency range () which fit best to the model
and observed AKR intensity profiles. The obtained results confirm the former
SWAVES antenna analysis by rheometry and numerical simulations. A final set of
antenna parameters is recommended as a basis for evaluations of the SWAVES
data
Erratic Jet Wobbling in the BL Lacertae Object OJ287 Revealed by Sixteen Years of 7mm VLBA Observations
We present the results from an ultra-high-resolution 7mm Very Long Baseline
Array (VLBA) study of the relativistic jet in the BL Lacertae object OJ287 from
1995 to 2011 containing 136 total intensity images. Analysis of the image
sequence reveals a sharp jet-position-angle swing by >100 deg. during
[2004,2006], as viewed in the plane of the sky, that we interpret as the
crossing of the jet from one side of the line of sight to the other during a
softer and longer term swing of the inner jet. Modulating such long term swing,
our images also show for the first time a prominent erratic wobbling behavior
of the innermost ~0.4mas of the jet with fluctuations in position angle of up
to ~40 deg. over time scales ~2yr. This is accompanied by highly superluminal
motions along non-radial trajectories, which reflect the remarkable
non-ballistic nature of the jet plasma on these scales. The erratic nature and
short time scales of the observed behavior rules out scenarios such as binary
black hole systems, accretion disk precession, and interaction with the ambient
medium as possible origins of the phenomenon on the scales probed by our
observations, although such processes may cause longer-term modulation of the
jet direction. We propose that variable asymmetric injection of the jet flow;
perhaps related to turbulence in the accretion disk; coupled with hydrodynamic
instabilities, leads to the non-ballistic dynamics that cause the observed
non-periodic changes in the direction of the inner jet.Comment: Accepted for Publication in The Astrophysical Journal. 11 pages, 6
figures, 4 tables. High resolution images on figure 1 and complete tables 1
and 2 may be provided on reques
Emission and propagation of Saturn kilometric radiation: magneto-ionic modes, beaming pattern and polarization state
The Cassini mission crossed the source region of the Saturn kilometric
radiation (SKR) on 17 October 2008. On this occasion, the Radio and Plasma Wave
Science (RPWS) experiment detected both local and distant radio sources, while
plasma parameters were measured in situ by the magnetometer (MAG) and the
Cassini Plasma Spectrometer (CAPS). A goniopolarimetric inversion was applied
to RPWS 3-antenna electric measurements to determine the wave vector k and the
complete state of polarization of detected waves. We identify broadband
extraordinary (X) as well as narrowband ordinary (O) mode SKR at low
frequencies. Within the source region, SKR is emitted just above the X mode
cutoff frequency in a hot plasma, with a typical electron-to-wave energy
conversion efficiency of 1% (2% peak). The knowledge of the k-vector is then
used to derive the locus of SKR sources in the kronian magnetosphere, that
shows X and O components emanating from the same regions. We also compute the
associated beaming angle at the source theta'=(k,-B) either from (i) in situ
measurements or a model of the magnetic field vector or from (ii) polarization
measurements. Obtained results, similar for both modes, suggest
quasi-perpendicular emission for local sources, whereas the beaming pattern of
distant sources appears as a hollow cone with a frequency-dependent constant
aperture angle: theta'=75{\deg}+/-15{\deg} below 300kHz, decreasing at higher
frequencies to reach theta'(1000kHz)=50{\deg}+/-25{\deg}. Finally, we
investigate quantitatively the SKR polarization state, observed to be strongly
elliptical at the source, and quasi-purely circular for sources located beyond
approximately 2 kronian radii. We show that conditions of weak mode coupling
are achieved along the ray path, under which the magneto-ionic theory
satisfactorily describes the evolution of the observed polarization
Quasi-simultaneous multi-frequency observations of inverted-spectrum GPS candidate sources
Gigahertz-Peaked Spectrum (GPS) sources are probably the precursors of local
radio galaxies.Existing GPS source samples are small (<200). It is necessary to
extend the availabe sample of the Gigahertz-Peaked Spectrum (GPS) and High
Frequency Peaker (HFP) sources in order to study their nature with greater
details and higher statistical significance. A sample of 214 radio sources,
which were extracted from the SPECFIND catalog and show an inverted radio
spectrum, were observed quasi-simultaneously at 4.85, 10.45, and 32GHz with the
100-m Effelsberg radio telescope. Using the VLBA calibrator survey (VCS) we
have investigated the parsec-scale morphology of the sources. About 45% of the
sources in our sample are classified as GPS or HFP candidates. We add 65 new
GPS/HFP candidates to existing samples. We confirm the expected tendency that
HFP are more compact on milliarcsecond scale than the 'classical' GPS sources,
which peak at lower frequencies. The data mining of the SPECFIND database
represents a promising tool for the discovery of new GPS/HFP sources.Comment: 16 pages, 7 figures, accepted for publication in A&
Climate Change Impacts on Iowa, 2010
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Supernovae 2016bdu and 2005gl, and their link with SN 2009ip-like transients: another piece of the puzzle
Supernova (SN) 2016bdu is an unusual transient resembling SN 2009ip. SN
2009ip-like events are characterized by a long-lasting phase of erratic
variability which ends with two luminous outbursts a few weeks apart. The
second outburst is significantly more luminous (about 3 mag) than the first. In
the case of SN 2016bdu, the first outburst (Event A) reached an absolute
magnitude M(r) ~ -15.3 mag, while the second one (Event B) occurred over one
month later and reached M(r) ~ -18 mag. By inspecting archival data, a faint
source at the position of SN 2016bdu is detectable several times in the past
few years. We interpret these detections as signatures of a phase of erratic
variability, similar to that experienced by SN 2009ip between 2008 and
mid-2012, and resembling the currently observed variability of the luminous
blue variable SN 2000ch in NGC 3432. Spectroscopic monitoring of SN 2016bdu
during the second peak initially shows features typical of a SN IIn. One month
after the Event B maximum, the spectra develop broad Balmer lines with P Cygni
profiles and broad metal features. At these late phases, the spectra resemble
those of a typical Type II SN. All members of this SN 2009ip-like group are
remarkably similar to the Type IIn SN 2005gl. For this object, the claim of a
terminal SN explosion is supported by the disappearance of the progenitor star.
The similarity with SN 2005gl suggests that all members of this family may
finally explode as genuine SNe, although the unequivocal detection of
nucleosynthesised elements in their nebular spectra is still missing.Comment: Submitted to MNRAS on April 10, 2017; re-submitted on June 23
including suggestions from the referee. 24 pages, 12 figures, 5 table
Wideband dynamic radio spectra of two ultra-cool dwarfs
A number of radio-loud ultra-cool dwarf (UCD ) stars exhibit both continuous broadband and highly polarized pulsed radio emission. In order to determine the nature of the emission and the physical characteristics in the source region, we have made multi-epoch, wideband spectral observations of TVLM 0513-46 and 2M 0746+20. We combine these observations with archival radio data to fully characterize both the temporal and spectral properties of the radio emission. The continuum spectral energy distribution can be well modeled using gyrosynchrotron emission from mildly relativistic electrons in a dipolar field. The pulsed emission exhibits a variety of time-variable characteristics, including frequency drifts, frequency cutoffs, and multiple pulses per period. For 2M 0746+20 we determine a pulse period consistent with previously determined values. We modeled locations of pulsed emission using an oblique rotating magnetospheric model with beamed electron-cyclotron maser ( ECM) sources. The bestfit models have narrow ECM beaming angles aligned with the local source magnetic field direction, except for one isolated burst from 2M 0746+20. For TVLM 0513-46, the best-fit rotation axis inclination is nearly orthogonal to the line of sight. For 2M0746+20 we found a good fit using a fixed inclination i = 36°, determined from optical observations. For both stars the ECM sources are located near feet of magnetic loops with radial extents 1.2Rs-2.7Rsand surface fields 2.2-2.5 kG. These results support recent suggestions that radio over-luminous UCDs have a global "weak field" non-axisymmetric magnetic topologies
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